feat(lib): switch to non-blocking (asynchronous) IO

BREAKING CHANGE: This breaks a lot of the Client and Server APIs.
  Check the documentation for how Handlers can be used for asynchronous
  events.

src/header/common/access_control_allow_credentials.rs

@@ -1,7 +1,7 @@
 use std::fmt::{self, Display};
 use std::str;
 use unicase::UniCase;
-use header::{Header, HeaderFormat};
+use header::{Header};
 
 /// `Access-Control-Allow-Credentials` header, part of
 /// [CORS](http://www.w3.org/TR/cors/#access-control-allow-headers-response-header)
@@ -62,9 +62,7 @@ impl Header for AccessControlAllowCredentials {
         }
         Err(::Error::Header)
     }
-}
 
-impl HeaderFormat for AccessControlAllowCredentials {
     fn fmt_header(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.write_str("true")
     }
@@ -86,4 +84,4 @@ mod test_access_control_allow_credentials {
     test_header!(not_bool,     vec![b"false"], None);
     test_header!(only_single,  vec![b"true", b"true"], None);
     test_header!(no_gibberish, vec!["\u{645}\u{631}\u{62d}\u{628}\u{627}".as_bytes()], None);
-}
+}